genetics chapt 16

The lac operon of E. coli controls the transcription of three genes needed in lactose metabolism: the lacZ gene, which encodes β- galactosidase; the lacY gene, which encodes permease; and the lacA gene, which encodes thiogalactoside transacetylase. The lac operon is negative inducible: a regulator gene produces a repres- sor that binds to the operator site and prevents the transcription of the structural genes. The presence of allolactose inactivates the repressor and allows the transcription of the lac operon.

...

• constitutive gene

A gene that is not regulated and is expressed continually.

competent cell

Capable of taking up DNA from its environment (capable of being transformed).

colinearity

Concept that there is a direct correspondence between the nucleotide sequence of a gene and the continuous sequence of amino acids in a protein.

• operator

DNA sequence in the operon of a bacterial cell. A regulator protein binds to the operator and affects the rate of transcription of structural genes.

• regulatory element

DNA sequence that affects the transcription of other DNA sequences to which it is physically linked.

• structural gene

DNA sequence that encodes a protein that functions in metabolism or biosynthesis or that has a structural role in the cell.

There are two basic types of transcriptional control: negative and positive. In negative control, when a regulatory protein (repressor) binds to DNA, transcription is inhibited; in positive control, when a regulatory protein (activator) binds to DNA, transcription is stimulated. Some operons are inducible; transcription is normally off and must be turned on. Other operons are repressible; transcription is normally on and must be turned off.

For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator. a. Negative control in a repressible operon / Inactive repressor b. Positive control in a repressible operon / Active activator c. Negative control in an inducible operon / Active repressor d. Positive control in an inducible operon / Inactive activator

• domain

Functional part of a protein.

Gene expression can be controlled at any of a number of points along the molecular pathway from DNA to protein, including DNA or chromatin structure, transcription, mRNA processing, RNA stability, translation, and posttranslational modification.

Functionally related genes in bacterial cells are frequently clus- tered together as a single transcriptional unit termed an operon. A typical operon includes several structural genes, a promoter for the structural genes, and an operator site to which the product of a regulator gene binds.

• regulator gene

Gene associated with an operon in bacterial cells that encodes a protein or RNA molecule that functions in controlling the transcription of one or more structural genes.

• regulatory gene

Gene associated with an operon in bacterial cells that encodes a protein or RNA molecule that functions in controlling the transcription of one or more structural genes.

• positive control

Generegulationinwhichthebindingofaregulatoryprotein to DNA stimulates transcription (the regulatory protein is an activator).

In bacteria, gene regulation maintains internal flexibility, turning genes on and off in response to environmental changes. In multicellular eukaryotic organisms, gene regulation brings about cellular differentiation.

Genes are DNA sequences that are transcribed into RNA. Regulatory elements are DNA sequences that are not transcribed but affect the expression of genes. Positive control includes mechanisms that stimulate gene expression, whereas negative control inhibits gene expression.

Gene expression can be controlled at different levels, including the alteration of gene structure, transcription, mRNA processing, RNA stability, translation, and posttranslational modification. Much of gene regulation is through the action of regulatory proteins binding to specific sequences in DNA.

Genes in bacterial cells are typically clustered into operons—groups of functionally related structural genes and the sequences that control their transcription. Structural genes in an operon are transcribed together as a single mRNA molecule.

gene regulation

Mechanisms and processes that control the phenotypic expression of genes.

• inducible operon

Operon or other system of gene regulation in which transcription is normally off. Something must take place for transcription to be induced, or turned on.

• repressible operon

Operon or other system of gene regulation in which transcription is normally on. Something must take place for transcription to be repressed, or turned off.

• regulator protein

Produced by a regulator gene, a protein that binds to another DNA sequence and controls the transcription of one or more structural genes.

• allosteric protein

Protein that changes its conformation on binding with another molecule.

• negative control

See supercoiling.Coiled tertiary structure that forms when strain is placed on a DNA helix by overwinding or underwinding of the helix. An overwound DNA exhibits positive supercoiling; an underwound DNA exhibits negative supercoiling.

• operon

Set of structural genes in a bacterial cell along with a common promoter and other sequences (such as an operator) that control the transcription of the structural genes.

• coordinate induction

Simultaneous synthesis of several enzymes that is stimulated by a single environmental factor.

What is the difference between a structural gene and a regulator gene?

Structural genes encode proteins; regulator genes control the transcription of structural genes.

• corepressor

Substance that inhibits transcription in a repressible system of gene regulation; usually a small molecule that binds to a repressor protein and alters it so that the repressor is able to bind to DNA and inhibit transcription.

• inducer

Substance that stimulates transcription in an inducible system of gene regulation; usually a small molecule that binds to a repressorprotein and alters that repressor so that it can no longer bind to DNA and inhibit transcription.

n negative control, a repressor protein binds to DNA and inhibits transcription. In positive control, an activator protein binds to DNA and stimulates transcription. In inducible operons, transcription is normally off and must be turned on; in repressible operons, transcription is normally on and must be turned off.

The lac operon of E. coli is a negative inducible operon. In theabsence of lactose, a repressor binds to the operator and prevents the transcription of genes that encode β-galactosidase, permease, and transacetylase. When lactose is present, some of it is converted into allolactose, which binds to the repressor and makes it inactive, allowing the structural genes to be transcribed.